1. Field of the Invention
The present invention relates to an optical fiber device comprising at least two kinds of optical fibers which are fusion-spliced to each other, and a method of making the same.
2. Related Background Art
Connector connection and fusion-splicing have been known to connect two kinds of optical fibers to each other. In general, the fusion-splicing is more often utilized for connecting optical fibers since the splice loss between the optical fibers is lower therein than in the connector connection. In this specification, an optical fiber device is an optical member including first and second optical fibers which are fusion-spliced to each other, whereas the size and length of the first and second optical fibers are not restricted. Examples of the optical fiber device include an optical transmission line constituted by a positive-dispersion optical fiber and a negative-dispersion optical fiber fusion-spliced to the positive-dispersion optical fiber; an optical transmission line constituted by a first optical fiber having a greater mode field diameter and a second optical fiber having a smaller mode field diameter fusion-spliced to the first optical fiber successively arranged in this order along the signal propagating direction; a dispersion-compensating module constituted by a dispersion-compensating optical fiber and a standard single-mode optical fiber fusion-spliced to one end or both ends of the dispersion-compensating optical fiber; an optical member constituted by a first optical fiber, in which a diffraction grating is formed by modulating the refractive index in a part of its light-propagating region, and a second optical fiber fusion-spliced to the first optical fiber; and the like.
When the first and second optical fibers constituting the optical fiber device have mode field diameters substantially identical to each other at a signal wavelength, e.g., 1.55 xcexcm or 1.3 xcexcm, before fusion-splicing in particular, the splice loss at the fused point of the optical fiber device is low. When the first and second optical fibers have mode field diameters different from each other before fusion-splicing, however, the splice loss at the fused point of the optical fiber device becomes greater.
Therefore, the vicinity of the fused point between the first and second optical fibers is heat-treated such that their mode field diameters coincide with each other after the fusing step in the latter case, so as to restrain the splice loss at the fused point from increasing. Namely, in this heating step, each of the first and second optical fibers is partly heated within a predetermined area including the fused point, so that impurities, e.g., Ge and F, added to each optical fiber (mainly composed of silica glass) are dispersed, whereby the difference in mode field diameter between the first and second optical fibers is lowered near the fused point. When the difference in mode field diameter between the fusion-spliced optical fibers is lowered as such, the splice loss at the fused point in the optical fiber device can be reduced.
In the technique disclosed in Japanese Patent Application Laid-Open No. HEI 6-18726, for example, the first and second optical fibers within a predetermined area including their fused point are heated with a small-size electric furnace in the heating step after the fusing step so as to attain the highest temperature of 1500xc2x0 C. to 1700xc2x0 C. and a marginal temperature (at both ends of the small-size electric furnace) of 900xc2x0 C. In the technique disclosed in Japanese Patent Application Laid-Open No. HEI 4-260007, on the other hand, the first and second optical fibers within a predetermined area including their fused point are heated with a micro torch in the heating step after the fusing step so as to attain the highest temperature of 1300xc2x0 C. to 1500xc2x0 C. Each of these publications shows a mode field diameter distribution within a predetermined area including the fused point after the heating step.
The inventors studied conventional techniques such as those mentioned above and, as a result, have found a problem as follows. Namely, in an optical fiber device constructed by fusion-splicing optical fibers having mode field diameters different from each other, the splice loss at the fused point between the optical fibers may not fully be reduced even if the heating step is carried out after the fusing step.
In order to overcome the problem mentioned above, it is an object of the present invention to provide an optical fiber device having a structure in which the ratio of change in mode field diameter of each optical fiber near the fused point is appropriately controlled such that the splice loss at the fused point is fully reduced, and a method of making the same.
The optical fiber device according to the present invention comprises first and second optical fibers, fusion-spliced to each other, having respective mode field diameters different from each other at a signal wavelength, e.g., 1.55 xcexcm or 1.3 xcexcm. In particular, in order to lower the difference in mode field diameter between the first and second optical fibers, the vicinity of the fused point between the first and second optical fibers is heat-treated after fusion-splicing. At a position separated from the fused point by a distance L, the first optical fiber has a first mode field diameter D1(L), whose minimum value is D10. On the other hand, at a position separated from the fused point by the distance L, the second optical fiber has a second mode field diameter D2 (L), whose minimum value is D20. The minimum values D10, D20 of the first and second mode field diameters refer to the respective mode field diameters of the first and second optical fibers at a signal wavelength (e.g., 1.55 xcexcm or 1.3 xcexcm) before fusion-splicing, i.e., mode field diameters excluding the vicinity of fused point where the mode field diameter changes. The xe2x80x9cmode field diameterxe2x80x9d in this specification, when simply mentioned as it is, refers to the mode field diameter before fusion-splicing, i.e., the minimum value of the first and second mode field diameters.
In particular, the inventors have found it preferable that, in the vicinity of the fused point between the first and second optical fibers, each of the maximum value of the ratio of change in the first mode field diameter (D1(L1)xe2x88x92D1(L2))/(L2xe2x88x92L1) between given two points respectively separated by distances L1 and L2 ( greater than L1) toward the first optical fiber from the fused point between the first and second optical fibers and the maximum value of the ratio of change in the second mode field diameter (D2(L1)xe2x88x92D2(L2))/(L2 xe2x88x92L1) between given two points respectively separated by distances L1 and L2 ( greater than L1) toward the second optical fiber from the fused point between the first and second optical fibers be 4.0 xcexcm/nm or less.
When the ratios of change in the first and second mode field diameters in the vicinity of the first and second optical fibers are controlled as such, the splice loss of the first and second optical fibers can effectively be reduced.
The ratios of change in the first and second mode field diameters in the vicinity of the fused point may also be controlled with reference to the first and second mode field diameters D1(0) and D2(0) at the fused point.
Namely, when the difference between the minimum value D10 of the first mode field diameter and the minimum value D20 of the second mode field diameter is 2 xcexcm or more, the optical fiber device preferably satisfies the following conditions:
D1(L)xe2x88x92D10xe2x89xa60.1 xcexcm (where Lxe2x89xa75 mm),
D2(L)xe2x88x92D20xe2x89xa60.1 xcexcm (where Lxe2x89xa75 mm),
(D1(0)xe2x88x92D1(2))/2xe2x89xa61.5 xcexcm/mm,
(D2(0)xe2x88x92D2(2))/2xe2x89xa61.5 xcexcm/mm,
(D1(0)xe2x88x92D1(3))/3xe2x89xa62.5 xcexcm/mm, and
(D2(0)xe2x88x92D2(3))/3xe2x89xa62.5 xcexcm/mm.
Also, when the difference between the minimum value D10 of the first mode field diameter and the minimum value D20 of the second mode field diameter is 2 xcexcm or more, the optical fiber device preferably satisfies the following conditions:
D1(L)xe2x88x92D10xe2x89xa60.1 xcexcm (where Lxe2x89xa75 mm),
D2(L)xe2x88x92D20xe2x89xa60.1 xcexcm (where Lxe2x89xa75 mm),
xe2x80x83(D1(0)xe2x88x92D1(2))/2xe2x89xa61.0 xcexcm/mm, and
(D2(0)xe2x88x92D2(2))/2xe2x89xa61.0 xcexcm/mm.
Further, when the difference between the minimum value D10 of the first mode field diameter and the minimum value D20 of the second mode field diameter is 2 xcexcm or more, the optical fiber device preferably satisfies the following conditions:
D1(L)xe2x88x92D10xe2x89xa70.1 xcexcm (where Lxe2x89xa65 mm), and
D1(L)xe2x88x92D10xe2x89xa60.1 xcexcm (where Lxe2x89xa75 mm).
When the difference between the minimum value D10 of the first mode field diameter and the minimum value D20 of the second mode field diameter is 2 xcexcm or less, by contrast, the optical fiber device preferably satisfies the following conditions:
D1(L)xe2x88x92D10xe2x89xa60.1 xcexcm (where Lxe2x89xa73 mm),
D2(L)xe2x88x92D20xe2x89xa60.1 xcexcm (where Lxe2x89xa73 mm),
(D1(0)xe2x88x92D1(1))/1xe2x89xa61.5 xcexcm/mm, and
(D2(0)xe2x88x92D2(1))/1xe2x89xa61.5 xcexcm/mm.
Also, when the difference between the minimum value D10 of the first mode field diameter and the minimum value D20 of the second mode field diameter is 2 xcexcm or less, the optical fiber device preferably satisfies the following conditions:
D1(L)xe2x88x92D10xe2x89xa70.1 xcexcm (where Lxe2x89xa61.5 mm), and
D1(L)xe2x88x92D10xe2x89xa60.1 xcexcm (where Lxe2x89xa72.0 mm).
The ratios of change in the first mode field diameter D1 (L) and second mode field diameter D2 (L) are appropriately controlled according to the difference between the minimum value D10 of the first mode field diameter (the mode field diameter before fusion-splicing) and the minimum value D20 of the second mode field diameter (the mode field diameter before fusion-splicing) as in the foregoing, whereby the splice loss at the fused point between the first and second optical fibers is effectively lowered.
Though there is a possibility that the splice loss may not fully be reduced when one of the minimum value D10 of the first mode field diameter and the minimum value D20 of the second mode field diameter is 2 xcexcm or more but 7 xcexcm or less, i.e., when the mode field diameter is relatively small in the first and second optical fibers before fusion-splicing, the splice loss at the fused point between the first and second optical fibers is fully reduced if the ratios of change in the first and second mode field diameters are controlled as mentioned above in the optical fiber device according to the present invention.
Though mismatching is more likely to occur between the first and second mode field diameters when one of the minimum value D10 of the first mode field diameter and the minimum value D20 of the second mode field diameter is 10 xcexcm or more but 14 xcexcm or less, i.e., when the mode field diameter is relatively large in the first and second optical fibers before fusion-splicing, the splice loss at the fused point between the first and second optical fibers is fully reduced if the ratios of change in the first and second mode field diameters are controlled as mentioned above in the optical fiber device according to the present invention.
The method of making an optical fiber device according to the present invention is characterized in that first and second optical fibers having respective mode field diameters different from each other are fusion-spliced, and then the vicinity of the fused point between the first and second optical fibers is heat-treated so as to control the ratio of change in mode field diameter in the vicinity of the fused point. Specifically, the method of making an optical fiber device according to the present invention comprises a fusing step of fusion-splicing one end of the first optical fiber and one end of the second optical fiber to each other, and a heating step of heating a predetermined region including the fused point between the first and second optical fibers after the fusing step.
In particular, it is preferred that, when the difference in mode field diameter between the first and second optical fibers (the difference in mode field diameter before fusion-splicing) is 2 xcexcm or more, the first and second optical fibers be partly heated in the heating step such that the difference between the highest and lowest temperatures in a region having a length of 4 mm centered at the fused point between the first and second optical fibers becomes 100xc2x0 C. or less (thus heating the vicinity of the fused point between the first and second optical fibers). More preferably, in the heating step, the first and second optical fibers are partly heated such that a position separated by 1.0 mm or less from the fused point between the first and second optical fibers toward one of the first and second optical fibers attains the highest temperature.
When the difference in mode field diameter between the first and second optical fibers is 2 xcexcm or less, by contrast, it is preferred that the first and second optical fibers be partly heated in the heating step such that the difference between the highest and lowest temperatures in a region having a length of 2 mm centered at the fused point between the first and second optical fibers becomes 100xc2x0 C. or less (thus heating the vicinity of the fused point between the first and second optical fibers). More preferably, in the heating step, the first and second optical fibers are partly heated such that a position separated by 0.5 mm or less from the fused point between the first and second optical fibers toward one of the first and second optical fibers attains the highest temperature.
In the method mentioned above, the temperature distribution in the vicinity of the fused point between the first and second optical fibers is appropriately controlled according to the difference in mode field diameter between the first and second optical fibers (the difference between the minimum value D10 of the first mode field diameter and the minimum value D20 of the second mode field diameter) in the heating step after the fusing step, whereby an optical fiber device having a lower splice loss at the fused point is obtained.
Preferably, in the method of making an optical fiber device according to the present invention, a flame formed by supplying a flammable gas and an oxygen gas to a micro torch (burner) is utilized in the heating step so as to heat a predetermined region near the fused point between the first and second optical fibers. It is also preferred that an electric heater be utilized so as to heat a predetermined region near the fused point between the first and second optical fibers. Each of these heating methods makes it possible to appropriately control the temperature distribution in the vicinity of the fused point between the first and second optical fibers.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.